$-\dfrac{3}{6} + \dfrac{7}{8} = {?}$
$ = - {\dfrac{3 \times 4}{6 \times 4}} + {\dfrac{7 \times 3}{8 \times 3}} $ $ = - {\dfrac{12}{24}} + {\dfrac{21}{24}} $ $ = - \dfrac{{12} + {21}}{24} $ $ = \dfrac{9}{24}$